The present invention relates to optical pickup devices. More particularly, the present invention relates to an optical pickup device that directs a laser beam of sufficient intensity for signal recording onto an optical recording medium to record and/or reproduce a signal onto/from a plurality of types of optical recording media.
Optical disks of approximately 1.2 mm in thickness to read out information using a semiconductor laser such as a CD-ROM (Compact Disk-Read Only Memory) are proposed. By carrying out focus servo and tracking servo on the objective lens for pickup with respect to this type of optical disk, a laser beam is directed to a pit train on the signal recording plane from which a signal is reproduced.
A CD-R (Compact Disk-Recordable) is available that has a recording density identical to that of the CD and that allows recording only once. A laser beam of 780 nm in wavelength is employed in recording and reproducting signals thereof.
Recently, the density is further increased to record a motion picture for a long period of time. For example, a DVD (Digital Video Disk) that records 4.7 Gbytes of information on one plane of an optical disk having a diameter of 12 cm that is identical to that of the CD-ROM is commercially available. The thickness of a DVD is approximately 0.6 mm. By fixing these planes together, 9.4 Gbytes of information can be recorded in one disk.
Attention is focused on a magneto-optical recording medium as a rewritable recording medium of great storage capacity and high reliability. The magneto-optical recording media are now applied as computer memories and the like. Standardization of a magneto-optical recording medium having a storage capacity of 6.0 Gbytes (AS-MO (Advanced Storage Magneto Optical Disk) standard) is in progress to be provided for actual usage. This magneto-optical recording medium has the signal reproduced by the MSR (Magnetically Induced Super Resolution) method. More specifically, a laser beam is projected to transfer the magnetic domain of the recording layer of the magneto-optical recording medium to a reproduction layer and also forming a detection window in the reproduction layer to allow detection of only the transferred magnetic domain. The transferred magnetic domain is detected from the formed detection window. A laser beam of 600-700 nm in wavelength is employed for recording and/or reproducing a signal onto and/or from the magneto-optical recording medium.
It is expected that there will be the coexistence of CDs, CD-Rs, DVDs and magneto-optical recording media in the future. The need arises for an optical pickup device that can reproduce information from such optical disks and that can record a signal onto a recordable optical disk. WO 98/19303 discloses an optical pickup device that allows compatible reproduction between a CD-R and a DVD.
The proposed CD-R/DVD compatible pickup includes a semiconductor laser generating a laser beam of 635 nm in wavelength for reproduction of a DVD and a semiconductor laser generating a laser beam of 780 nm in wavelength for recording and reproduction of a CD-R. When a signal is to be recorded onto or reproduced from a CD-R using a laser beam of 780 nm in wavelength, the laser beam is diffracted and a desired diffracted light thereof, for example only the first order light, is introduced into the objective lens to collect light in order to correct aberration caused by difference in the thickness of the substrate.
Therefore, the zero order light or minus first order light could not be used effectively. There was a problem that a laser beam sufficient in intensity for recording could not be obtained at the signal recording plane of the CD-R.
An object of the present invention is to provide an optical pickup device capable of recording and/or reproducing a signal onto/from a plurality of types of optical disks of different thickness, suppressing laser beam loss to the minimum.
According to an aspect of the present invention, an optical pickup device recording and/or reproducing a signal onto/from a first optical disk and a second optical disk thicker than the first optical disk includes a light source, an objective lens, and an optical device. The light source generates a laser beam. The objective lens is located opposite to the first and second optical disks. The optical device is arranged between the light source and the objective lens to transmit the laser beam from the light source straightforwardly during recording or reproduction of the first optical disk, and bending substantially the entire laser beam from the light source and increasing the diameter thereof to guide the center portion of the laser beam towards the objective lens and the peripheral portion of the laser beam outside the objective lens during recording or reproduction of the second disk.
In the optical pickup device, the optical device bends substantially the entire laser beam from the light source so that only the center portion of the laser beam is guided to the objective lens during recording or reproduction of the second optical disk, so that most of the laser beam can be used effectively with the exception that the peripheral portion is lost. A signal can be recorded onto the first and second optical disks or a signal can be reproduced from the first and second optical disks while suppressing loss of the laser beam at the minimum.
Preferably, the optical device includes a first optical member and a second optical member. The first optical member has a first refractive index. The second optical member is in contact with the first optical member, and has the first refractive index during recording or reproduction of the first optical disk, and has a second refractive index differing from the first refractive index during recording or reproduction of the second optical disk. During recording or reproduction of the first optical disk, the entire optical device has the first refractive index. Therefore, the laser beam from the light source is transmitted straightforwardly. In contrast, the first and second optical members have different refraction indexes during recording or reproduction of the second optical disk. Therefore, the optical device diffracts or refracts the laser beam from the light source.
Further preferably, the light source generates a first laser beam having a first wavelength during recording or reproduction of the first optical disk, and generates a second laser beam having a second wavelength differing from the first wavelength during recording or reproduction of the second optical disk. The first optical member has a first refractive index for the first and second wavelengths. The second optical member has the first refractive index for the first wavelength and the second refractive index for the second wavelength. Since the refractive index of the second optical member changes according to the wavelength, the laser beam can be transmitted straightforwardly or bent without mechanical switching.
Also preferably, the first optical member includes a hologram formed to come into contact with the second optical member. Therefore, the optical device diffracts the laser beam by interference during recording or reproduction of the second optical disk.
Further preferably, the first optical member is arranged at the light source side. The second optical member is arranged at the objective lens side. The first refractive index is higher than the second refractive index. The hologram includes a plurality of annular projections formed concentrically. The pitch of the annular projections become smaller as towards the outer circumference. Therefore, the optical device diffracts the laser beam at a greater angle as towards the circumference.
Preferably, each of the annular projections has a triangular cross section radially. Therefore, the optical device can diffract the incident laser beam in a desired direction without generating 0 order or xe2x88x921 order diffracted light.